Described herein are curable inks, and more particularly, radiation curable solid ink compositions comprising solid monomers and reactive wax for direct to substrate imaging applications, particularly their use in ink jet printing.
In general, solid inks (also referred to as phase change inks or hot melt inks) are in the solid phase at ambient temperature, but exist in the liquid phase at the elevated operating temperature of an ink jet printing device. At the jet operating temperature, droplets of liquid ink are ejected from the printing device and, when the ink droplets contact the surface of the recording substrate, either directly or via an intermediate heated transfer belt or drum, they quickly solidify to form a predetermined pattern of solidified ink drops. Phase change inks have also been used in other printing technologies, such as gravure printing.
Phase change inks for color printing typically comprise a phase change ink carrier composition which is combined with a phase change ink compatible colorant. A series of colored phase change inks can be formed by combining ink carrier compositions with compatible subtractive primary colorants. The subtractive primary colored phase change inks can comprise four component dyes, namely, cyan, magenta, yellow and black, although the inks are not limited to these four colors. These subtractive primary colored inks can be formed by using a single dye, a single pigment, a mixture of dyes, a mixture of pigments, or a combination thereof.
Solid inks typically used with ink-jet printers have a wax-based ink carrier, for example, a crystalline wax-based ink carrier. Such solid ink-jet inks provide vivid color images. In typical systems, the crystalline-wax inks are jetted onto a transfer member, for example, an aluminum drum, at temperatures of approximately 120 to about 140° C. The wax-based inks are heated to such high temperatures to decrease their viscosity for efficient and proper jetting onto the transfer member. The transfer member is typically at a temperature of about 60° C., so that the wax will cool sufficiently to solidify or crystallize. As the transfer member rolls over the recording medium, for example paper, the image comprised of wax-based ink is pressed into the paper.
However, the use of crystalline waxes places limitations on the printing process used for conventional solid inks, particularly if the inks are used in a direct to paper application. First, the printhead must be kept at a temperature of about 120° C. which can lead to a number of problems. At these high temperatures, dyes that are molecularly dissolved in the ink carrier are often susceptible to unwanted interactions leading to poor ink performance. For example, the dyes may be susceptible to thermal degradation, dye diffusion from the ink into the paper or other substrate, leading to poor image quality and showthrough, leaching of the dye into other solvents making contact with the image, leading to poor water/solvent-fastness. Further, for direct to paper applications it is desirable to heat the image after printing to achieve dot gain. In addition, for some substrates, the optimum spreading of the ink drops is difficult to achieve. Moreover, when the printhead is cooled and re-warmed, the resulting contraction and expansion of the ink requires a purge cycle to achieve optimum printhead performance. Particularly, the robustness (for example, smear resistance) of current inks can be insufficient for many potential applications.
Radiation curable inks generally comprise at least one curable monomer, a colorant, and a radiation activated initiator that initiates polymerization of curable components of the ink. Radiation-curable inks can be employed in ink jet printing systems. Radiation-curable phase-change inks are known as well, as disclosed in, for example, U.S. Pat. Nos. 7,153,349, 7,259,275, 7,270,408, 7,271,284, 7,276,614, 7,279,506, 7,279,587, 7,293,868, 7,317,122, 7,323,498, 7,384,463, 7,449,515, 7,459,014, 7,531,582, 7,538,145, 7,541,406, 7,553,011, 7,556,844, 7,559,639, 7,563,489, 7,578,587, 7,625,956, 7,632,546, 7,674,842, 7,681,966, 7,683,102, 7,690,782, 7,691,920, 7,699,922, 7,714,040, 7,754,779, 7,812,064, and 7,820,731, the disclosures of each of which are totally incorporated herein by reference. Radiation-curable phase change inks can exhibit additional desirable characteristics such as improved hardness and scratch-resistance and improved adhesion to various substrates. Radiation-curable gel inks can also exhibit advantages in that dot spread of the ink can be controlled, the ink does not bleed excessively into the substrate.
While currently available ink compositions are suitable for their intended purposes, a need remains for a new type of solid ink that is capable of being printed via the piezoelectric ink jet printing process. There is further a need for ink compositions that can be processed at lower temperatures and with lower energy consumption, have improved robustness, have improved jetting reliability and latitude, and do not require an intermediate transfuse drum and high pressure fixing. In addition, a need remains for a new type of solid ink composition that exhibits desirably low viscosity values at jetting temperatures, generates images with improved look and feel characteristics, generates images with improved hardness and toughness characteristics, and that is suitable for a number of commonly used substrates. There is further a need for a solid ink composition that can ensure, to the extent that toxic or otherwise hazardous compounds are used in such compositions, that migration, evaporation or extraction of such materials from this new type of ink be controlled or ameliorated. When used in certain applications, for example food packaging, and direct to paper printing, it is desirable to reduce the amount of or eliminate altogether extractable species present, for example to meet environmental, health and safety requirements. There is also a need for solid inks that can be jetted at temperatures below those required for conventional solid inks—in some specific embodiments 10, 20, 30° C., or more below conventional jetting temperatures. Further, there is a need for solid inks that exhibit low shrinkage upon cooling from the melt. Additionally, there is a need for solid inks that exhibit little or no odor. A need also remains for solid inks that can be transfused at temperatures of about 70° C. or lower. A need also remains for solid inks with improved lightfastness.